The difference between baking soda and washing soda is water and carbon dioxide. Seriously. Baking soda's chemical makeup is NaHCO3 (1 sodium, 1 hydrogen, one carbon, and 3 oxygen molecules). Washing soda's chemical makeup is Na2CO3 (2 sodium, 1 carbon, and 3 oxygen molecules). When baking soda is heated up to high temperatures, it breaks down to become washing soda, water steam, and carbon dioxide.

All you need to do to turn ordinary baking soda into washing soda is to pour baking soda in a wide baking dish and bake on 400-degrees Fahrenheit until it loses its luster and becomes a little grainy—a commenter remarked that a batch of 6 tablespoons took around half an hour.Photo by Nature's Nurture Blog